Presently, 3GPP, which is an organization for standardizing technical specifications of third-generation mobile phones, is promoting the international standardization of LTE (Long Term Evolution). LTE is for significantly improving the performance of 3G systems. Specifically, LTE is for implementing high-speed transmission in mobile communications, to establish an environment for providing high-level services such as high-speed video distribution. Furthermore, LTE is expected to be a system for efficient usage of frequencies. Furthermore, in 3G systems, voice communication and data communication are provided by different infrastructures. However, in LTE, all communication infrastructures for mobile phones are integrated in an IP based infrastructure. Accordingly, instead of the conventional mobile system including a circuit switching type infrastructure, the LTE system is expected to provide a mobile system that has been completely turned into an IP-based system.
FIG. 1 illustrates an example of an LTE network. In the LTE network illustrated in FIG. 1, a user terminal (UT) and base stations (eNodeB, eNB) are wirelessly connected, and the base stations are connected to aGW (E-UTRAN Access Gateway). The aGW is connected to IASA (Inter AS Anchor), which is an anchor device in an access system. The IASA includes SAE-A (SAE Anchor) that is an anchor device with respect to an external network, and 3GPP-A (3GPP Anchor) that is an anchor device with respect to a network specified by 3GPP. Furthermore, aGW and IASA are connected to HSS (Home Subscriber Server). Furthermore, IASA is connected to another network via PCRF (Policy & Charging Rule Function).
Real-time data in a mobile system (speech call, video streaming, etc.) is expected to be turned into IP-based data by protocol stacks as illustrated in FIGS. 2 and 3. FIG. 2 illustrates an example of a speech call protocol stack (for PSTN). FIG. 3 illustrates an example of a speech call protocol stack (in own AS). That is to say, FIG. 2 illustrates the connection of an LTE system and an existing circuit switching system, and FIG. 3 illustrates the connection of plural LTE systems.
Real-time data (typically speech call) is transferred by RTP/UDP/IP, similar to VoIP used in a typical wired IP network. The overhead increases because headers of protocols are attached to the data. Thus, the bandwidth in a wireless zone is limited. Accordingly, for the purpose of improving the usage efficiency of a wireless zone, the header is preferably compressed. In the LTE system, the function of compressing the header is expected to be implemented by ROHC (RObust Header Compression: RFC 3095), which is included in the function of the PDCP (Packet Data Convergence Protocol: TS36.323) that is terminated between the base station device (eNodeB) and the user terminal.